• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.



Thank for the reaction. The stages are C-coupled and they're not leaking. After the first stage and cap however the second half of the paraphase is driven (signal applied to the grid, attached to ground via 100 k resistor). Indeed somehow here is the result a -4 Volts which passes to the grids of the output tubes. It is not the grid that is drawing current however.

If I take the ecc82 (very first stage) out of the amplifier (and effectively make the amplifier open loop) all problems are gone. It has to be a oscillation (I have no scope). But if this is true, all AI200 must have this problem!